Lithium ion secondary batteries and other types of non-aqueous electrolyte secondary battery are becoming increasingly important as power sources fitted to vehicles and power sources for personal computers, hand-held terminals and the like. In particular, lithium ion secondary batteries, which are lightweight and achieve high energy density, are preferred as vehicle-mounted high output power sources.
In non-aqueous electrolyte secondary batteries such as lithium ion secondary batteries, some of the non-aqueous electrolyte decomposes during charging, and a coating film formed from this decomposition product, that is, a SEI (Solid Electrolyte Interface) film, can be formed on the surface of a negative electrode active material (for example, natural graphite particles). The SEI film has the function of protecting the negative electrode active material, but is formed by consuming charge carriers (for example, lithium ions) in the non-aqueous electrolyte. That is, if a charge carrier is immobilized on the SEI film, the charge carrier can no longer contribute to the battery capacity. Therefore, a large quantity of SEI film is a possible cause of a deterioration in capacity retention rate (a deterioration in cycle characteristics).
In order to address this problem, a variety of additives have been incorporated in non-aqueous electrolytes in order to form a stable coating film in advance on the surface of a negative electrode active material instead of a SEI film. In non-aqueous electrolyte secondary batteries provided with coating films formed by decomposition of additives, it is known that reactive resistance is lowered and battery life is improved. For example, Patent Literature 1 discloses a non-aqueous electrolyte for a secondary battery, wherein the non-aqueous electrolyte contains a compound having an oxalate complex as an anion (hereinafter referred to as an “oxalate complex compound”).